A wavelength division multiplexing (WDM) communication system is an optical communication system using a wavelength multiplexing technology. The WDM communication system is a technology capable of drastically increasing the transmission capacity by bundling signal light of an equal-grid wavelength defined by standardization and transmitting the signal light in one optical fiber. A tunable laser is mainly used as a light source of the WDM communication system. In the tunable laser used in the WDM communication system, it is desirable to precisely control an oscillation wavelength so as to match the grid wavelength, and generally, a wavelength locker is used.
A wavelength locker of the related art includes, for example, a first beam splitter, a second beam splitter, an etalon, a first photodetector, and a second photodetector. From the output light of the tunable laser, a part of the light branched by the first beam splitter is further split into two light beams by the second beam splitter. One of the light beams passes through a periodic filter such as an etalon, and the transmitted light intensity thereof is monitored by the first photodetector. The light intensity of the other light beam is monitored by the second photodetector as it is. The ratio of the monitored values of the first photodetector and the second photodetector (corresponding to the transmittance of the etalon) is fed back so that the ratio matches the value of the transmittance of the etalon at a desired wavelength, and the wavelength is controlled.
Since a wavelength locker is constituted of a large number of parts as described above, the size thereof is large and is an obstacle to miniaturization of a tunable laser device which incorporates a wavelength locker in a single package together with a tunable laser. On the other hand, for example, Japanese Laid-open Patent Publication No. 2015-68854 discloses a technology of miniaturizing the size of the wavelength locker by concentrating the beam splitters and a delay interference waveguide serving as an alternative to the etalon as one optical waveguide device. The delay interference waveguide is constituted by splitting light into two and including a first optical waveguide and a second optical waveguide having different lengths. By propagating the branched light to the first optical waveguide and the second optical waveguide, respectively, and then coupling the propagated light again, the delay interference waveguide serves as a filter in which the intensity of the output light periodically varies with respect to the wavelength, corresponding to the optical waveguide length difference between the first optical waveguide and the second optical waveguide. The wavelength cycle of the delay interference waveguide is inversely proportional to the optical waveguide length difference between the first optical waveguide and the second optical waveguide. By adopting such a configuration, it is possible to remarkably reduce the size of the wavelength locker as compared with the case using a plurality of parts in the related art.
Japanese Laid-open Patent Publication No. 2015-60944, and Japanese Laid-open Patent Publication No. 2013-15728 are examples of the related art.
In a WDM communication system, a grid interval is mainly 50 GHz at present, and it is thought that the interval will shift to a narrower interval such as 6.25 GHz in the future. In the wavelength locker of 50 GHz for the grid interval, a periodic filter frequency of 50 GHz is appropriate, but in order to realize such a filter in a delay interference waveguide, it is desirable to set an optical path length difference between the two optical waveguides approximately 6 mm. Depending on the material of the optical waveguide, a desirable optical waveguide length difference becomes very long as 1.5 mm to 4 mm. Further, in the future, considering shifting to a narrower grid interval, it is expected that a desirable optical waveguide length will be further increased. Therefore, in a case where a delay interference waveguide is used as a periodic filter for a wavelength locker, there is a limit to miniaturization of the size thereof, which is an obstacle to miniaturization of a tunable laser device including a wavelength locker.